Display devices that include display panels such as liquid crystal panels are used in portable information terminals, electronic devices, and television devices include display devices that include display panels. Examples of the portable information terminals include mobile phones, smartphones, and tablet-type notebook personal computers. Examples of the electronic devices include computers. In such display devices, the liquid crystal panels do not emit light and thus lighting devices, that is, backlight units are required. The backlight units are generally categorized into a direct type and an edge-light type according to structures. Each of the direct type backlight units includes top-emitting LEDs mounted on a substrate on which a conductive pattern is formed. Light emitted by the LEDs uniformed through an optical member such as a diffuser plate is supplied to a liquid crystal panel.
The substrate used in such a backlight unit includes mounting areas in which the LEDs are mounted. White resists are formed in areas of the substrate other than the mounting areas to improve insulating properties and reflectivity. Alternatively, a reflection sheet including holes in areas corresponding to the LED mounting areas and having high reflectivity is disposed.
Reflectivity of white resists is usually 90% or lower. Therefore, light is not effectively used and a sufficient level of brightness cannot be achieved for the backlight. To place the reflection sheet having the reflectivity higher than that of the white resists, it may be difficult to align the holes in the reflection sheet with the LEDs and fix the reflection sheet. Furthermore, the reflection sheet may be placed over the LEDs if thermal expansion and contraction of the reflection sheet occur. To resolve such problems, certain sizes of holes may be required. Because the reflectivity of a surface of the substrate exposed through the holes is low, the light cannot be effectively used. If the number of the LEDs is large (namely, the number of the holes is large), such a problem becomes larger. To reduce displacement of the reflection sheet due to the thermal expansion and contraction, the reflection sheet may be bonded to the substrate with an adhesive layer. However, the substrate on which the LEDs are mounted is not flat and thus a roller cannot be used. Therefore, it is difficult to bond the reflection sheet to the substrate with high accuracy without air bubbles. If a metal thin film is used for the reflection sheet, opening edges of holes in the metal thin film may contact solders that were provided for electrical connection for the LEDs and leak currents may occur. To reduce the leak current, the holes in areas in which the LEDs are mounted may be increased in size. However, the surface of the substrate having lower reflectivity may be exposed through the holes resulting in reduction of brightness.